A generic injection moulding machine has the usual components of a machine of this type, namely an injection unit with a plasticising screw and drive units to generate the movement of the injection unit itself and the injection and after-pressure stroke of the plasticising screw. Furthermore, a fixed and a moveable tool clamping plate are conventionally provided, on which the respective half moulds are attached. In a so-called three-plate machine, a further support plate for the mould closing unit displacing the moveable tool clamping plate can optionally be provided. The mould closing unit has a drive unit to generate the mould closing and opening movement of the moveable tool clamping plate and to apply the so-called tool closing force during injection. Finally, a workpiece ejector device with a corresponding drive unit is provided on a tool clamping plate.
Injection moulding machines were originally designed as hydraulic machines with respect to their drive units but for some time, certain drive axles have been controlled by electromechanical drive assemblies such as servomotors. So-called fully electric machines are also already conventional.
The present invention relates to injection moulding machines, in which at least one of the drive units is hydraulic and, accordingly, a hydraulic pump, a hydraulic cylinder and one or more control valves are provided to activate the hydraulic cylinder.
In the known hydraulic drive units, the movement speeds of this axle to be driven in each case of the injection moulding machine are controlled by proportional servo valves of control pumps. Conventional zero-one valves in this case realise switch-on or switch-off delays of 30 to 60 ms. The dynamics of control pumps are in similar time ranges. Servo valves with shorter reaction times are known but these are very expensive and, with reaction times in the range of 15 to 20 ms, also not yet optimal for the requirements of an injection moulding machine.
Machines of this type are distinguished by the fact that the precision and reaction speed of the machine are determined by the dynamics of the activation of the individual axes. For example, time delays of the hydraulic actuators during the switch over in the plasticising screw from injection pressure to after-pressure are disadvantageous. The dynamics of the screw movement have to be greatly reduced in the vicinity of this switch-over point in order to avoid pressure overshoots in the tool. The latter would cause so-called over-injections or “webs” in the region of the mould division plane, damage the injection moulding material itself and cause stresses therein, which can later lead to distortion of the product produced. Coinciding problems also occur in electromagnetic injection moulding machines, as the electric rotary drives used therein have inevitable mass moments of inertia and can also therefore not be switched between two operating states adequately rapidly and dynamically.